The present invention relates to a digital automobile telephone in which the BCCH (Broadcast Channel) receiving period is controlled on the basis of the field intensity of the received radio wave, thereby achieving stable reception and saving electric power.
In recent years, the demand for automobile telephones has been increasing, and from the viewpoint of effectively utilizing radio waves, TDMA (Time Division Multiple Access) automobile telephones have been developed as substitutes for the conventional FDMA (Frequency Division Multiple Access) automobile telephones.
The latter type is generally called "digital automobile telephone". Standards for the digital automobile telephone have already been provided (Radio System Development Center (Japan): RCR-27, 27A, and 27B). According to the standards, when a mobile station is waiting, it is required to receive BCCH containing notification information and PCH (Paging Channel) containing incoming information, which are constantly sent from a base station. Particularly, it is compulsory for the mobile station to receive BCCH periodically.
FIG. 1 is a block diagram showing an example of the arrangement of a conventional digital automobile telephone. In the figure, an operation unit 8 constitutes a telephone operating device. An input signal from the operation unit 8 is output to a system control unit 5 through an operation interface 7.
A call control unit 6 performs a call-out operation for a destination under the control of the system control unit 5. The call control unit 6 also performs a call-in operation for a call-in signal that is input thereto from a radio channel processing unit 1 through the system control unit 5.
A speech processing unit 2 performs the following operations under the control of the system control unit 5. That is, the speech processing unit 2 converts speech data into human speech and outputs it to a receiver 3, and it also converts human speech, which is input from a transmitter 4, into speech data and outputs it to the radio channel processing unit 1 so that the speech data is transmitted from an antenna 10.
However, when BCCH is periodically received according to the standard rules, the following problems may arise, depending upon the condition of radio waves:
Firstly, when the radio field intensity is high, or when the condition of reception is very good, BCCH information can be stably received, but yet the same content is received many times uselessly, resulting in a waste of electric power.
Secondly, when the radio field intensity is low, or when the condition of reception is poor, reception of BCCH information is unstable. Therefore, although the base station repeatedly sends the same content, erroneous information is received due to noise or other interference each time reception is performed.